The available methods for a fiber-reinforced plastic production include an autoclave molding process in which a reinforcing fiber base material is first impregnated with a resin to produce a prepreg, followed by heating and pressing it in an autoclave to produce moldings, and a RTM (resin transfer molding) process in which a reinforcing fiber base material free from resin impregnation is fed into the inner space of a molding die comprising a top mold and a bottom mold, followed by maintaining compression of the molding die by using, for instance, an oil hydraulic press, injecting a compressed resin into the inner space, and heat-curing the resin to produce moldings.
Also available is a VaRTM (vacuum assisted resin transfer molding) process in which a bagging film is used instead of the top mold to allow the pressure in the inner space closed by the bagging film to be reduced by vacuum aspiration to cause a resin to be injected into a reinforcing fiber base material by the pressure difference from atmospheric pressure, followed by heat curing to provide moldings. This process has been widely used for production of a fiber-reinforced plastic because it does not require compression equipment such as oil hydraulic press as described above to permit low cost production of moldings.
Assuming that Darcy's law holds for a resin impregnation into a reinforcing fiber base material by processes such as RTM and VaRTM, the flow rate v (m/s) of a resin is expressed as follows:v=−K·∇P/μ  (1)where K (m) denotes permeability, an index representing the easiness of impregnation into the reinforcing fiber base material with the resin, P (Pa) denotes the pressure of the resin, and μ (Pa·s) denotes the viscosity of the resin.
In this formula, ∇P represents the pressure gradient. As the value of permeability K (m) increases, it becomes easier to impregnate the reinforcing fiber base material with the resin.
It is seen that the impregnation distance of the resin is proportional to the permeability K (m) of the reinforcing fiber base material used and the pressure P (Pa) of the resin and inversely proportional to the viscosity μ (Pa·s) of the resin.
Accordingly, impregnation of a thick reinforcing fiber base material with a resin requires an increased resin injection pressure P (Pa) or a decreased resin viscosity. There is a limit, however, to reduction in resin viscosity, and it is actually necessary to increase the resin injection pressure P (Pa).
To maintain a high resin injection pressure P (Pa), it is necessary to perform compression by using a pressure device such as oil hydraulic press that can maintain a required inner space in a molding die that would not open while a resin is injected into the inner space.
On the other hand, the VaRTM process, which is performed under atmospheric pressure and requires no compression equipment, conventionally has the disadvantage that the resin impregnation thickness is limited because the resin injection pressure is restricted by atmospheric pressure (JP 2007-176163 A). To solve this problem, a process has been proposed in which a resin diffusion medium or resin passage is provide on both sides of a layered body (a layered body comprising a stack of a plurality of reinforcing fiber base materials) and a resin is injected into the layered body through both surfaces of the layered body so that the layered body can impregnated to an increased thickness (depth) with the resin (JP 2004-188965 A and JP 2008-179149 A).
However, both JP '965 and JP '149 fail to disclose a method of preventing the resin injected through the resin diffusion medium or resin passage from being discharged through a resin suction port (vacuum aspiration port) after taking short cuts instead of serving to impregnate the reinforcing fiber base materials. Thus, the processes have a problem of discharge of resin through a resin suction port before impregnating the reinforcing fiber base materials and also have a problem of unimpregnated portions being left in the reinforcing fiber base materials.
In addition, both JP '965 and JP '149 fail to disclose a method of preventing voids from being confined in the resin that is injected through both surfaces of the layered body into the inner part of the layered body. This results in a problem of confinement of voids in fiber-reinforced plastic moldings.
For the VaRTM process, JP 4432563 B describes a method of eliminating formation of resin-unimpregnated portions in reinforcing fiber base materials by preventing formation of shortcuts to the resin suction port. However, it deals only with injection of a resin from one side of a layered body and has no description about resin injection from both sides of a layered body which is necessary to impregnate thick reinforcing fiber base materials with a resin. If applied to the thick reinforcing fiber base materials, therefore, that method has a problem of an inability to impregnate the thick reinforcing fiber base materials with a resin, leading to unimpregnated portions left in the reinforcing fiber base materials.
It could therefore be helpful to provide a method of producing a fiber-reinforced plastic by RTM or VaRTM that can inject a resin into reinforcing fiber base materials, particularly a thick one having a thickness of 10 mm or more, without leaving resin-unimpregnated portions and depress the formation of voids in the reinforcing fiber base materials.
It could also be helpful to provide a method of producing a fiber-reinforced plastic that produces a fiber-reinforced plastic by VaRTM at minimized cost by eliminating the use of large pressing equipment such as oil hydraulic press, injecting resin into reinforcing fiber base materials without leaving unimpregnated portions, and preventing formation of voids in the reinforcing fiber base materials so that equipment cost is largely reduced as compared with the RTM process that uses a molding die comprising a top mold and a bottom mold and compression equipment such as oil hydraulic press.
There is further a need to provide a method of producing a fiber-reinforced plastic in which even when applied to the RTM process, which uses a molding die comprising a top mold and a bottom mold and compression equipment such as oil hydraulic press, thick reinforcing fiber base materials can be impregnated under a smaller pressure without leaving unimpregnated portions, thereby making it possible to use a simplified die and smaller size compression equipment, reduce the required equipment cost, and provide fiber-reinforced plastics at minimized prices.
Although the conventional method including a step of disposing a resin diffusion medium on both sides of a layered body and a step of injecting a resin from both sides into the layered body to impregnate it is an effective means of impregnating a thick layered body with resin, the conventional two side impregnation process lacks a means of preventing formation of shortcuts that lead the resin to a resin suction port and, consequently, the resin tends to take shortcuts to a resin suction port lower in flow path resistance instead of penetrating in the thickness direction into thick reinforcing fiber base materials that are higher in flow path resistance, leading to the problem of formation of resin-unimpregnated portions in the layered body. Even if a step of injecting a resin from both sides of a layered body is added, the process still has a problem of confinement of voids between resin layers, resulting in voids remaining in a molded fiber-reinforced plastic.
Thus, there is a need to provide a method of producing a fiber-reinforced plastic that performs resin impregnation of a thick layered body while preventing the formation of shortcut resin flow paths to a resin suction port (vacuum aspiration port) to depress the formation of resin-unimpregnated portions and at the same time depressing formation of voids in the layered body during the injection of the resin and the impregnation of the layered body.